1. Field of the Invention
This invention relates to a heat sealing apparatus for sealing thermoplastic films, and in particular to a portable heat sealer having a plurality of adjacent segmented heating elements, positioned along the underside of a heated sealing bar, which operates under microprocessor control.
2. Description of Related Art
Thermoplastic films are conventionally sealed by heat applied in a number of ways including hot wires, ultrasonically vibrated heads, and radiant energy. Such sealing devices normally utilize opposed jaw members with one of the jaw members being heated and the other jaw member provided with a resilient facing. Generally the heated jaw of the sealer consists of an aluminum bar with an internal heating element which is known in the art as a “sealer bar”. After the pouch or other item constructed from thermoplastic films is placed between the jaws, a mechanism is provided which is operator actuatable to clamp the jaws and start the sealing cycle. The period of time the jaws are closed and the amount of heat applied are typically predetermined. An example of this type of sealer is shown in U.S. Pat. No. 4,378,266, issued Mar. 29, 1983 to Carl F. Gerken, which describes a lever used to actuate a jaw and set an electronic timer. However, imperfect seals occur due to non-uniform heating along the heated jaw.
Another type of sealer utilizes a heated wire in place of the sealer bar to bond thermoplastic films. Examples of such devices are taught in U.S. Pat. No. 3,792,770, issued Feb. 19, 1974 to Max Freeman, and U.S. Pat. No. 3,490,981, issued Jan. 20, 1970 to Frank Garrett Shanklin. While relatively inexpensive and relatively rapid in operation such method of sealing is not able to consistently produce strong liquid-tight hermetic seals. In some products, the presence of an imperfection or less than consistently perfect seal may be acceptable, for example, frozen foods packaged in plastic resin film pouches and L-frame seals used for shrink packaging. However, such inferior sealing is not acceptable in other situations where the seal made by a heated wire has relatively little strength and may easily burst from internal pressure.
All such prior art devices, including heated sealer bars and heated wires, have been subject to heating problems in which the thermoplastic films can be either overheated or under heated causing imperfect seals as well as problems of non-uniform heating along the length of the seal due to variations in the pressure and temperature along the heated sealing bar. This problem is particularly acute when a heated sealer bar is used to produce hermetic seals that are long and wide for thick, laminate, thermoplastic films.
The heat sealing of thermoplastic films requires that the material be elevated in temperature to a certain point and held under heat and pressure for a certain length of time in order to bring about an effective seal. If more heat than is necessary is used to cause bonding of the thermoplastic materials, they may scorch, and the use of too little heat may prevent the desired bonding. The former condition usually results in the accumulation of burnt materials on the heated sealing bar which interferes with the functioning of the sealer. In a similar manner, variations in pressure along the jaw member provided with a resilient facing can result in variations in seal quality. In summary, the possibility of overheating portions of the seal, while under heating other areas and variation in the pressure provided by the resilient facing, can cause seal imperfections such as pronounced seal weakness.
Previous attempts to address the issue of uniform heating of the sealer bar have included improvements upon a sheathed type electrical heater which utilizes magnesium oxide insulation as described in U.S. Pat. No. 1,869,140, issued Jul. 26, 1932 to W. F. Gelinas. Another example of this is a rod type heating device consisting of a bent longitudinal element configured in a manner to provide uniform heat in the center of the element as described in U.S. Pat. No. 4,349,727, issued Sep. 14, 1982, to John W. Churchill. Various other configurations of the rod type-heating element have been developed to produce more even heating including rod type heating elements with a triangular cross-section. An example of a triangular tubular heater of the prior art is the Chromalox Model TI, heart-shaped, cross-section heater manufactured by the Chromalox Division of Emerson Electric Company, of Pittsburg, Pa. There is also an example of increasing the wall thickness, weight and thermal conductivity of the heated sealing bar in an attempt to more evenly distribute the heat along its surface as described in U.S. Pat. No. 4,451,325, issued May 29, 1984, to Wilfried Bubenzer.
More recently, the heated sealing bar has been mounted on springs so that the entire heated sealing bar is movable inwardly and outwardly a certain amount to apply a constant pressure. This approach is described in U.S. Pat. No. 5,034,087, issued Jul. 23, 1991 to Stanley D. Denken.
The common deficiencies encountered with the above prior art teachings is that they require a high degree of mechanical complexity to achieve liquid tight, hermetic seals. The primary deficiency of all these devices is that they require heavy rigid construction. Another important deficiency is that they require considerable time to arrive at a sealing temperature due to their large thermal mass. Still another important deficiency is that they are not well suited for the lightweight and low cost design requirements of a portable heat sealer.
The problems with the heat sealers of the prior art are overcome by the heat sealer to be described in the following summary and description.